Heat exchangers of the type used to recover heat from gas turbine or gas/diesel engine exhaust gas are commonly designed with a bypass circuit situated external to the heat exchanger array and its casing, with the exhaust gas flow to the heat exchanger array circuit and the bypass circuit controlled by one or two flap valves or the like, such valves being known as dampers. Arrangements are known in which a single damper controls the flow through both circuits. Alternatively, two damper arrangements are known, in which one damper controls the flow through the heat exchanger array circuit and the other damper controls the flow through the bypass circuit. Both types tend to be heavy, bulky and complicated and when such dampers have been continuously modulated for continuously variable flow control reliability problems have been experienced. For example. with two damper arrangements, damage to engines has been caused by excessive back-pressure due to both dampers being closed at the same time, instead of one circuit always being open.